1. Field of the Invention
The present invention relates to an improved light receiving member which is highly sensitive to electromagnetic waves such as light (which herein means in a broad sense light such as ultraviolet rays, visible rays, infrared rays, X-rays, and .gamma.-rays). More particularly, the present invention relates to an improved light receiving member having a multi-layered light receiving layer with an enhanced concentration of hydrogen or/and halogen atoms in the vicinity of the interface of adjacent layers which is suitable as a photosensitive member for use in information processing devices such as electrophotographic copying machines and laser beam printers, as a photosensor, or as a solar cell.
2. Description of Related Background Art
For the photoconductive material to constitute an image-forming material for use in solid image pickup device or electrophotography, or to constitute a photoconductive material for use in image-reading photosensor, it is required to be highly sensitive, to have a high S/N ratio (photocurrent (Ip)/dark current (Id)), to have absorption spectrum characteristics suited for an electromagnetic wave irradiated, to be quickly responsive and to have a desired dark resistance. It is also required to be not harmful to living things, especially man, upon use.
As the photoconductive material which satisfies these requirements, there are known so-called amorphous silicon materials (the amorphous silicon material will be hereinafter referred to as "a-Si material"). It is known that a-Si materials are high in Vickers hardness and have a good durability. There are a number of proposals of applying a-Si materials in the preparation of electronic devices. For example, U.S. Pat. Nos. 4,265,991, 4,451,547, 4,552,824, and 4,507,375, and Offenlegungsschriftes Nos. 2746967 and 2855718 disclose use of a-Si materials in electrophotographic image-forming members. Further, Offenlegungsschrift No. 2933411 discloses use of a-Si materials in a photoelectric conversion image-reading device. Other than these, U.S. Pat. Nos. 4,461,819, 4,551,405, 4,557,990, 4,613,558, 4,359,512, and 4,359,514 disclose light receiving members having an a-Si light receiving layer with a stacked structure comprising two or more layers each having a different conductivity and wherein a depletion layer is formed in an interfacial region between adjacent layers. In addition, U.S. Pat. Nos. 4,394,425 and 4,394,426 disclose layer constitutions of improving the light receiving member comprising an a-Si material such that it can be designed at a relatively relaxed construction while maintaining the advantage of the a-Si material of exhibiting a high photosensitivity even when the dark resistance is low to a certain extent. The particulars of these layer constitutions include a manner of designing the light receiving member to be of a multi-layered structure having a barrier layer between a substrate and a light receiving layer (having a photoconductive layer) and a manner of designing the light receiving member to be of a multi-layered structure having a barrier layer over a light receiving layer (having a photoconductive layer). The "barrier layer" herein means a layer which functions to prevent a photocarrier from getting into the photoconductive layer from either the substrate side or the outermost layer side and to allow a photocarrier generated in the photoconductive layer upon the irradiation of an electromagnetic wave which mobilizes toward the substrate side, to move from the photoconductive layer side toward either the substrate side or the outermost layer side.
A number of electrophotographic image-forming members each comprising an a-Si material (hereinafter referred to as a-Si electrophotographic image-forming member or a-Si light receiving member) based on the above proposals have been commercialized. However, for any of the conventional a-Si electrophotographic image-forming members (the conventional a-Si light receiving members in other words), there are still some subjects which requires further improvements in terms of overall viewpoints including electrical, optical and photoconductive characteristics such as dark resistance, photosensitivity, photoresponsiveness, and the like, use-environmental characteristics such as moisture resistance, durability, and the like, and economic stability, in order to satisfy the requirements desired for a light receiving member used in the recent electrophotographic copying machines.
In recent years, a remarkable improvement has been made in the electrophotographic copying machine especially in terms of copying speed and durability upon repeated use over a long period of time. Particularly, there has been developed an improved electrophotographic copying machine which can operate at a higher process speed while exhibiting its image-reproducing performance without being deteriorated even upon repeated use over a long period of time. For such electrophotographic copying machine, there is a demand for improving the reliability of each constituent member thereof so that the maintenance work frequency can be reduced, in order to curtail the expenses required for the maintenance work. Other than this, there is another demand for further improving the electrophotographic copying machine so that it can attain a large volume image reproduction of high quality and high resolution at a high speed.
Along with this, there is an increased demand for providing an improved a-Si light receiving member which exhibits an improved charge retentivity and an improved sensitivity which is suitable for use in such electrophotographic copying machine.
In the case of repeatedly conducting the electrophotographic image-forming process comprising charging, exposure, developing and transfer steps at a higher speed in the electrophotographic copying machine using the conventional a-Si light receiving member (that is, the conventional a-Si electrophotographic photosensitive member), there often occurs a problem in that the a-Si light receiving member does not exhibit a photoresponsibility sufficient to follow the increased, image-forming process speed and because of this, it is difficult to stably and repeatedly obtain a high quality copied image at a higher speed. Particularly, in the case where a half-tone based original is subjected to repetitive reproduction at a high speed in the electrophotographic copying machine using the conventional a-Si light receiving member (the conventional a-Si electrophotographic photosensitive member), there is a tendency that the resulting copied images often have defects such as insufficiency in half-tone resolution, and unevenness in image density, which are found slightly in the case of copied images reproduced from a character original. Therefore, it is difficult to repeatedly obtain a high quality copied image which is equivalent to the half-tone original. This tendency is apparent in the case of using a half-tone original in a single color and with a uniform density in the entire area such as a photograph of a blue sky, a photograph of a single-colored wall of a building, or a single-colored paper, wherein the appearance of the above defects on the resulting copied images is apparent, especially in terms of unevenness in image density. This situation becomes significant as the image-forming speed is heightened.
Description will be made of this situation. That is, upon repeatedly conducting the electrophotographic image-forming process in the electrophotographic copying machine, the related image-forming parameters including the surface potential and surface temperature of the a-Si light receiving member are properly adjusted so as to repeatedly provide an identical desirable copied image in each repetition of the image-forming process by detecting these parameters by means of a sensor disposed in the copying machine and controlling them to predetermined respective values by means of a control mechanism disposed in the copying machine. In the case where the photoresponsibility of the a-Si light receiving member is insufficient to follow the image-forming process speed, the a-Si light receiving member after having been subjected to the electrophotographic image-forming process is barely returned to the original state which is completely free of the remainder of the previous latent image, wherein the values of the parameters of the a-Si light receiving member detected by means of the sensor are eventually varied. In this case, it is necessary to properly adjust the image-forming parameters of the a-Si light receiving member in each repetition of the image-forming process. Should this situation be continued over a long period of time, problems eventually occur in that it is difficult to continuously provide an identical desirable copied image, and a serious burden is imposed upon the control mechanism, sometimes resulting in shortening the machine main body life. Particularly, for the copied images provided upon repeating the image-forming process, there often appear image defects such as deficiency in minute line reproduction, appearance of white fogging (or white marks on half-tone copies), unevenness in image density, and the like, which are likely due to unevenness in charge retentivity and unevenness in photosensitivity of the a-Si light receiving member.
The appearance of these image defects is relatively marked in the case of reproducing a large copy volume at a higher speed using the large-sized high performance electrophotographic copying machine. Particularly, in the case of repeatedly conducting the image-forming process at a higher speed using the conventional a-Si light receiving member, the sensitivity exhibited by the a-Si light receiving member is insufficient to follow the image-forming process speed. Hence, there often occurs a problem in that the latent images formed on the a-Si light receiving member in the previous image-forming process still remain in the form of a half-tone, resulting in providing a ghost on a copied image obtained. In addition to this, there often occurs another problem such that, as so-called blank exposure is usually conducted to the a-Si light receiving member once having been subjected to the electrophotographic image-forming process to extinguish the surface charge in order to prevent a surface portion of the a-Si light receiving member corresponding to the interval between one copying paper sheet and the other copying paper sheet to be successively supplied, from being deposited with toner, the history of the previous blank exposure often remain to cause a so-called blank exposure memory on an image reproduced. The image obtained is accompanied by such blank exposure memory and is poor in uniformity in terms of density. (The above ghost and blank exposure memory will be hereinafter collectively expressed by an inclusive term "photomemory".)
These problems are more liable to occur in the case of repeatedly conducting the image-forming process at a higher speed using the conventional a-Si light receiving member having a multi-layered photoconductive layer comprising a plurality of layers each having a different optical band gap or having a function-divided photoconductive layer comprising a charge generation layer and a charge transportation layer, since there is a tendency that not only the photosensitivity but also the mobility of a photocarrier at the interface between the adjacent layers becomes insufficient and the charge retentivity is lowered as the image-forming process speed is increased.
As above described, conventional a-Si light receiving members are problematic in that the photoresponsibility and the mobility of a photocarrier become insufficient and the appearance of photomemory are apparent as the image-forming process speed is increased.
Incidentally, there is a demand for providing a compact electrophotographic copying machine which can operate at a high speed. The a-Si light receiving member (the a-Si electrophotographic photosensitive member) to be used in such compact electrophotographic copying machine is accordingly required to be of a small size so that it can be suitable for use therein. In this case, the image-forming process speed is eventually increased to a level which is markedly higher than that used in the ordinary electrophotographic copying machine with the use of the ordinary a-Si light receiving member, in order to attain the same copy volume in the conventional electrophotographic copying machine. The occurrence of the above problems becomes more significant in this case.
Now, in order to avoid the occurrence of the foregoing problems in the case of repeatedly conducting the image-forming process at a higher speed using the conventional a-Si light receiving member, it is necessary to take measures such as enlarging the charger and/or of effectively conducting the charging within a short period of time, and in addition, it is necessary to employ an exposure mechanism having a high power-outputting performance. These factors lead to not only raising the production cost of an electrophotographic copying machine but also making the electrophotographic copying machine larger in size.